Congenital malformations, or structural birth defects, are now the leading cause of infant mortality in the US and Europe [1, 2]. Of the congenital malformations, congenital heart disease (CHD) is the most common [1, 2]. Clinical and genetic studies have provided direct evidence for the role of mutations in T-box transcription factors as causes of a number of CHDs. Central to this proposal mutations in the T-box gene Tbx20 are causative in a range of cardiac abnormalities including dilated cardiomyopathy, atrial septal defects, or mitral valve disease, while upregulation of TBX20 expression has been reported in patients with Tetralogy of Fallot (i.e., pulmonary outflow tract obstruction, ventricular septal defect, overriding aortic root and right ventricular hypertrophy) [3-7]. While TBX20 is an essential transcription factor for heart development and its disease relevance is well established, there are many critical questions unanswered about the mechanism of how TBX20 functions. We do not understand what proteins complex with TBX20 during different stages of cardiac development and homeostasis, how these interactions regulate TBX20's choice of distinct transcriptional targets at different times, or how these interactions function to activate and/or repress target gene transcription. To this end, our labs recently initiated a directed proteomic-based approach to identify proteins that function in association with TBX20. These studies demonstrate that TBX20 functions through distinct complexes that associate with TBX20 at defined periods of cardiomyocyte differentiation. Collectively, this work led to the central hypothesis that TBX20 function and thus its suites of target genes are regulated during cardiac development through changes in the components of the TBX20 interactome.